Interface switching apparatus and switching control method

ABSTRACT

In order to share a device among a plurality of hosts, a switching section is adapted to select one of the hosts by switching and connect it to the device by a serial interface. The switching section can be controlled from the particular host through at least an interface cable. When the switching section switches the host to be connected with the device, power continues to be supplied to the device. Further, a connection recognition protocol to be transmitted from the device when beginning to supply power to the device is transmitted from the switching section. In this way, a quick switching operation free of protocol contradictions is made possible without causing any hardware resetting of the device.

This is a continuation application of U.S. Ser. No. 08/964,603, filedNov. 5, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to an interface switching apparatus and aswitching control method wherein a plurality of information processingsystems (hereinafter referred to as the hosts) having a serial interfacecapable of dynamic insertion and removal can share at least a peripheraldevice (hereinafter referred to as the device) having the same serialinterface. The present invention also relates to a device which can beshared by a plurality of hosts. The present invention further relates tothe interface switching apparatus and the switching control method inwhich a USB (Universal Serial Bus) is applied to the serial interface.

The use of the USB is extending as a serial interface for connecting theperipheral units (devices) such as a keyboard and a mouse to theinformation processing systems (hosts).

The USB specification is briefly described in “UNIVERSAL SERIAL BUSSPECIFICATION version 1.0 (Jan. 15, 1996), pp.27-28, available from theU.S. USB Implementers Forum (URL on internet is http://www.usb.org/).

Each USB host is basically connected with one USB device. The use of aUSB hub having the port repeater function, however, makes it possible toconnect a plurality of USB devices to each USB host.

In addition to USB, a serial interface called IEEE1394 is findingapplications. IEEE1394, like USB, is capable of dynamic insertion andremoval and can be added through a hub. The data transfer rate of theUSB is about 12 MBits/sec at maximum, whereas IEEE1394 specified by IEEEhas a data transfer rate as high as 400 MBits/sec.

In recent years, a system called “a cluster” is extending as a systemfor improving the reliability of information processing systems, inwhich each of a plurality of information processing systems monitors theoperations of the other information processing systems and ready toswitch various processes to a normal system in case of a fault. Thecluster system is configured of a plurality of servers, and in order toreduce the installation space, the devices such as the monitors, thekeyboards and the mice (hereinafter referred to as “the consoledevices”) are desirably shared in use.

Conventional means for sharing the console devices is switching amonitor signal or a keyboard signal by a switch. The journal “ASCII/V,March Issue” published by ASCII in February 1996, p. 192 discloses akeyboard switch for performing a switching operation by depressing aspecified combination of keys on the keyboard.

SUMMARY OF THE INVENTION

Among the various console devices, the interfaces of the keyboard andthe mouse have been increasingly using the USB. According to the USBspecification, however, connection of a device to a plurality of USBhosts is not permitted. For a given USB bus, for example, only one USBhost can exist at a time.

The first problem, therefore, is that in sharing a console among acluster system or among a plurality servers, a USB console device suchas a keyboard or a mouse cannot be connected to a plurality of hosts ata time.

A possible solution to this method is to switch the route of connectionbetween a USB device and a USB host by normal switching means as in theprior art. In the USB, however, the dynamic insertion and removalfunction requires the process of disconnecting and connecting the USBsignal each time the switch is operated. As in the normal dynamicinsertion and removal operation, therefore, a hardware reset control ofa device is executed based on the USB specification. The second problemis that a simple switching circuit consumes a considerable time before aUSB device becomes usable each time of switching by the hardware resetoperation accompanying the dynamic insertion and removal operation.

The third problem is that the switching of a console has no relationwith the main host of a cluster system in the conventional interfaceswitching means using the manual switching operation. In the case wherea fault is detected in a given host, for example, the console cannot beswitched in response to the fail-over (the operation of switching aprocess at the time of a fault) of the cluster to inform the manager ofthe fault quickly.

The fourth problem is that if the operation of the interface switchingmeans is to be interlocked with a fail-over of a cluster, the host isrequired to perform the switching control operation using a differentcontrol signal in view of the fact that the conventional keyboardinterface lacks any specification of communicating such a switchingcommand.

The fifth problem of the conventional manual interface switching meansis that even an input not connected to a host could be selected. In asystem constituting a three-input switch having only two hosts, forexample, nothing is displayed on the monitor when switched to theremaining one input so that a keyboard entry is invalid veryinconveniently in the absence of a host to communicate with. The user isrequired to be always conscious of this limitation.

The sixth problem is that if a USB switch obviating the first or secondproblem described above is available, a monitor signal as well as thekeyboard and the mouse is actually required to be switched. The monitorsignal, however, is a video signal or governed by the IEEE1394specification described above and requires a different interfaceswitching means.

The seventh problem is that when a given USB keyboard is connected toanother USB host having a different architecture using some switchingmeans, the switching of the key code is required.

A first object of the present invention is to provide a USB switchingapparatus, and a USB device which can be connected to a plurality of USBhosts and thus can be shared among a plurality of the USB hosts.

A second object of the invention is to provide a USB switching apparatuswhich can reduce the time required before a USB device becomes usableafter starting to switch the USB.

A third object of the invention is to provide a USB switching apparatuswhich can switch the connection of a USB host in response to an commandfrom the host in the case where a fault occurs in the USB host in acluster configuration or the like.

A fourth object of the invention is to provide a USB switching apparatusin which a switching command can be issued from a USB host without anynew control interface between the USB host and the USB switchingapparatus.

A fifth object of the invention is to provide a USB switching apparatusin which the connection is not switched to a connection system withwhich any USB host is not connected.

A sixth object of the invention is to provide the USB switchingapparatus described above, which is capable of switching the interfacesother than the USB at the same time.

A seventh object of the invention is to provide a USB keyboard capableof being connected to a USB host of a different architecture.

In order to achieve the above-mentioned objects, the present inventioncomprises the means described below.

The means for solving the first problem is a USB switch having aninterface with a plurality of USB hosts, in which an arbitrary one ofthe USB hosts is selected to establish a connection route with a USBdevice.

The means for solving the second problem is a USB switch having aninterface with a plurality of USB hosts, configured in such a mannerthat during the period when a connection route established between anarbitrary host and a device is switched to another arbitrary host, thedevice continues to be supplied with power and the host with which aconnection is to be newly established is supplied with the same signalas when the device is attached dynamically. Specifically, the devicecontinues to be supplied with power and therefore is not reset when thehost is switched. Also, the host can recognize the switching of thedevice correctly by receiving a pseudo-dynamic insertion signal.

The means for solving the fourth problem is configured in such a mannerthat a switching control command is issued from a host to a USB switchwhich receives the switching control command as a USB device forperforming the switching operation. As a result, the host can freelycontrol the connection route, and a new control signal is not requiredto be added for connecting the host and the USB switch.

The third problem can be solved by the host issuing the above-mentionedswitching control command upon detection of a fault in the clustercontrol software.

The means for solving the fifth problem is so configured that thehost-side interface of a USB switch recognizes a host-side connection orthe switching power on, and the switching control of a host isinvalidated in the case where the connection is not established or poweris not supplied on host side.

The means for solving the sixth problem is configured in such a mannerthat the host switching control signal for the USB switch is used alsofor switching other interfaces.

The means for solving the seventh problem is configured in such a mannerthat a key code table is provided in the USB keyboard and switchedaccording to the architecture of the USB host connected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a hardware configuration of a USB hostselector according to a first embodiment.

FIG. 2 is a flowchart showing the operation of a switching controlsection of the USB host selector of FIG. 1.

FIG. 3 is a block diagram showing a hardware configuration of amulti-host connection USB keyboard according to a second embodiment.

FIG. 4 is a block diagram showing a hardware configuration of amulti-host connection USB keyboard according to a third embodiment.

FIG. 5 is a perspective view of wirings using a USB host selectoraccording to a fourth embodiment.

FIG. 6 is a block diagram showing the connection of the switchingcontrol section of the USB host selector of FIG. 5.

FIG. 7 is a flowchart showing the operation of the switching controlsection of FIG. 6.

FIG. 8 is a perspective view showing wirings using a USB host selectoraccording to the fourth embodiment.

FIG. 9 is a block diagram showing a hardware configuration of the USBhost selector according to a fifth embodiment.

FIG. 10 is a block diagram showing a hardware configuration of theswitching section of FIG. 9 according to the fifth embodiment.

FIG. 11 is a time chart showing the state of each signal in the processof switching the switching section.

FIG. 12 is a flowchart showing the operation of the control software forFIG. 9.

FIG. 13 is a flowchart showing the operation of the switching controlsection of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the invention will be explained with reference tothe accompanying drawings. The present invention is not limited to thefirst embodiment.

The USB switching circuit, the USB switching apparatus and the USB hostselector described below are all a USB interface switching apparatus.The USB cable, on the other hand, is a transmission cable including fourlines including two USB signal lines, a SUB power line and a USB groundline. The two USB signal lines are differential signal lines, one ofthem being called a D+ signal line and the other a D− signal line. Byway of explanation, the state in which communication is possible betweena USB host and a USB device by a USB interface is called the USBconnection (first embodiment).

FIG. 1 is a block diagram showing an example of a hardware configurationof a USB host selector 100 according to the first embodiment of theinvention.

The USB host selector 100 connects a plurality of USB hosts 103 a to 103d and a plurality of USB devices 104 a to 104 c to each other thereby toestablish a connection route between the USB hosts 103 a to 103 d andthe USB devices 104 a to 104 d. The internal configuration of the USBhost selector 100 is described below.

Numerals 101 a to 101 g designate USB connectors and numeral 105 a USBhub for repeating the USB signal to a plurality of the USB devices 104.

The portion defined by the dotted line represents a USB switchingapparatus 106 and constitutes a feature of the present invention.Numeral 107 designates a USB bus interface connected to a USB hub 105.The USB bus interface 107 has the functions of interpreting a switchingcontrol command issued from any of the USB hosts 103 in terms of the USBand delivering it to a switching control section 110. The internalportion of the USB switching apparatus 106 having the switching functionis shown as a switching circuit 108, and the switching control section110 is included in the switching circuit 108.

The internal portion of the switching circuit 108 actually switches thesignal is a switching section 109. This switching section 109 can be amechanical switch or an electrical switch (such as a switch including aMOS transistor).

Numerals 112 a to 112 d designate host connection detectors fordetecting the connection and the power-on state of the USB hosts 103. Adetection result signal 113 is applied to the switching control section110.

In FIG. 1, the switching control operation is performed in any of themethods described below. In one of the methods, the USB hosts 103 sendout a switching command and the switching control section 110 interpretsthe command through the USB bus interface 107. Another method is thedirect switching by the user operating a manual change-over switch 111.Still another method consists in controlling not to switch to any of theUSB hosts 103 not connected or not powered on according to the result ofdetection by the host connection detectors 112.

In the method of switching by operating the manual change-over switch111 directly, the switching control section 110 performs prioritycontrol as to the switching control command issued from the USB hosts103.

FIG. 2 shows an example of a flowchart showing the operation of theswitching control section 110.

Step 201 executed after switching on power checks whether at least oneUSB host 103 correctly switched on is connected to any one of the USBconnectors 101 a to 101 d.

Step 202 switches the switching section 109 to an arbitrary one of validUSB hosts.

In step 203, the USB bus interface 107 and the USB devices 104 a to 104c connected to the USB connectors 101 e to 101 g are initialized inresponse to an initialization command (reset signal) transmitted from aspecified USB host 103 with the route thereof established by switchingafter power is switched on. The detection and initialization of the USBdevices is described in “UNIVERSAL SERIAL BUS SPECIFICATION version 1.0(Jan. 15, 1996), pp. 165-171”.

Upon completion of execution of the above-mentioned steps, the routebetween a console device and a host is established as viewed from theuser, and the keyboard entry and the monitor display are made possible

Assuming that a switching command is issued by a USB host 103 connected,it is detected in step 204 and the process proceeds to step 206. In thecase where the user issues a switching command manually, on the otherhand, the process proceeds from step 205 to 206.

Step 206 checks whether the USB host 103 to be switched is correctlyconnected, and step 207 switches the switching section 109.

After switching in step 207, the process returns not to step 204 but tostep 203. This is by reason of the fact that the switching of the USBhosts 103 requires the initialization of the USB devices 104.

Steps 208 to 210 represent a flow for the case in which the USB host 103that has thus far been connected is deactivated or disconnected for somereason or other. In the case shown, the detection by the host connectiondetectors 112 is notified by interruption to the switching controlsection 110.

If a valid USB host 103 exists other than the USB host 103 that hasestablished its route (step 209), a command is issued to switch to thevalid USB host (step 210). This process permits an invalid console to bedisconnected quickly.

The first embodiment described above with reference to FIGS. 1 and 2 isan example of a solution to the first, fourth and fifth problems.

A second embodiment of the invention will be described below.

FIG. 3 is a block diagram showing a hardware configuration of amulti-host connection USB keyboard 400 according to the secondembodiment of the invention.

The feature of this multi-host connection USB keyboard 400 is that itincludes the switching circuit 108 of FIG. 1 and also a key scan codestorage area 405 for meeting the requirements of a plurality of USBhosts of different architecture as desired. This multi-host connectionUSB keyboard 400 will be briefly described below.

A key scan section 403 reads the depression of a key switch 404, andsends key entry information to a keyboard control circuit 402. Thekeyboard control circuit 402 converts the key scan code using a key scancode set corresponding to the type of a particular USB host 103, andsends it to the particular USB host 103 through a USB bus interface 401and a switching circuit 108.

The key scan code set storage area 405 holds a table showing key entriesand corresponding scan codes. The architecture of the USB host is notspecified for the USB, and therefore the USB may be connected to a USBhost 103 of a different architecture by the USB switching circuit 10.

Once a scan code is selected at the time of establishing the route withan arbitrary USB host in the switching circuit 108, a corresponding keyscan code can be output regardless of the architecture of the USB host103 connected. For selecting a scan code, a USB connector 101corresponding to each scan code is determined in advance, or the type ofeach USB host is determined by some means at the time of switching onpower, or a corresponding scan code can be transmitted as a part of theinitialization command from the USB hosts 103 each time of the switchingoperation of the switching circuit 108.

A third embodiment of the invention will be described below.

FIG. 4 is an example of a derivation of the multi-host connection USBkeyboard 400 shown in FIG. 3. The multi-host connection keyboardaccording to this embodiment is different from the multi-host connectionUSB keyboard shown in FIG. 3 and therefore is designated illustrativelyby numeral 500.

The feature of this invention lies in a plurality of keyboard controlcircuits 502 a to 502 c and a host select circuit 504 by which akeyboard entry can be output to a plurality of arbitrary USB hosts 103.In other words, a key entry on the keyboard can be broadcast.

The host select circuit 504 has built therein a host select register505. In accordance with the setting of this host select register 505,the key entry is transmitted to the keyboard control circuits 502. Eachof the keyboard control circuits 502 accesses the key scan code settingstorage area 405 and thus out puts a key code corresponding to thearchitecture of the USB host 103 connected to the route of theparticular keyboard control circuit 502.

The host select control section 503 built in each keyboard controlcircuit 502 sets the host select register 505 as to whether a key codeis to be transmitted to the route of the particular host select controlsection 503. The command for this operation is the same command that thecorresponding USB host 103 has issued to the particular keyboard controlcircuit 502 through the corresponding USB bus interface 501.

The host select circuit 504 transmits the key entry information receivedfrom the key scan section 403 to the keyboard control circuit 502associated with the bit value “1”of the host select register 505 throughthe corresponding signal line 507. No key entry information is sent to akeyboard control circuit 502 having a bit value “0”. In the case where aplurality of bit values of the host select register 505 are “1”, the keyentry information is transmitted simultaneously to the correspondingkeyboard control circuits 502.

Each keyboard control circuit 502 monitors whether the corresponding USBhost 103 is correctly connected. Upon detection of a disconnection by akeyboard control circuit 502, the host select circuit control section503 associated therewith sets the corresponding bit value of the hostselect register 505 of the host select circuit 504 to “0”automatically,thus improving the operating convenience of the user. The connection ordisconnection is displayed on a key transmission host display LED 506.

A fourth embodiment of the invention will be described below.

FIG. 5 shows an embodiment for solving the sixth problem. The USB hostselector is designated by numeral 600 for express distinction.

The feature of this invention resides in that the USB host selector isconnected to a VGA selector 601 and an IEEE1394 host selector 602.

The USB host selector 600 establishes a route by switching theconnection between the USB hosts 603 a to 603 c on the one hand and thekeyboard 610 accorded to USB and the mouse 611 accorded to USB on theother hand. The VGA selector 601 switches the VGA signals 605 outputfrom the USB hosts 603 and establishes a connection route to the display609 accorded to VGA. The IEEE1394 host selector 602 switches theIEEE1394 cables 606 connected with the USB hosts 603, and thusestablishes a route with an IEEE1394 device. In FIG. 5, a floppy diskdrive 612 is assumed to be connected as such a device.

The operation of the USB host selector 600 is different from that of theUSB host selector 100 explained above in that output signal lines 607,608 are led out from the USB host selector 600. The other aspects of theoperation basically remain the same and will not be described.

FIG. 6 is a diagram showing the relation between the input and outputsignals of the switching control section 620 built in the USB hostselector 600. The switching control section 620 corresponds to theswitching control section 110 shown in FIG. 1.

The feature of the switching control section 620 shown in FIG. 6 isdifferent from that of the switching control section 110 in that theoutput signal lines 607 and 608 are led out from the switching controlsection 620. The signals from the signal lines 607 and 608 are outputbased on the switching of the USB hosts 603 for controlling theswitching operation of the VGA selector 601 and the IEEE1394 hostselector 602.

The internal configuration of the VGA selector 601 is similar to that ofthe conventionally known switching device and will not be described. Theinternal configuration of the IEEE1394 host selector 602, on the otherhand, can be considered basically identical to that of the USB hostselector 600 except for the difference in the interface specification.

FIG. 7 is a diagram showing an example of the flow of the switchingoperation of the USB host selector 600.

Steps 700 and 701 judge whether the operator has depressed the manualchange-over switch 111 or has received a switching control command fromany of the USB hosts 103. The process then proceeds to step 703.

Step 703 judges whether or not the IEEE1394 host selector 602 isconnected. In the case where the IEEE1394 is connected, step 704requests the IEEE1394 control software on the USB hosts 603 to prohibitthe use of the device accorded to IEEE1394 in order to prevent theswitching operation from being performed while data is being transmittedor received by the floppy disk drive 612.

In step 705, the USB control software operating on the USB hosts judges,by inquiry to the IEEE1394 control software or otherwise, whether thefloppy disk drive 612 is can be switched. If the judgement is that thefloppy disk drive 612 can be switched, then step 706 sends out aswitching control command through the signal line 608 to the IEEE1394host selector 602, and step 707 confirms the transmission through thesignal line 608.

Step 708 judges whether or not the VGA selector 601 is connected, and ifconnected, step 709 sends out a switching control command through thesignal line 607 to the VGA selector 601.

Step 710 switches the USB host 603 connected to the keyboard 610accorded to USB and mouse 611 accorded to USB.

FIG. 8 shows an example of a configuration in which a VGA selector 800and an IEEE1394 host selector 801 are used as the USB devices 104,respectively. The USB host selector 100 has built therein the USB hub105 shown in FIG. 1, and is adapted to transmit a switching controlcommand to the VGA selector 800 and the IEEE1394 host selector 801. Thisconfiguration eliminates the need of defining a special protocol for thesignal lines 607, 608 in FIG. 6, and a configuration is possible basedon the commercially-available multi-purpose LSI used for the USB.

The configurations shown in FIGS. 5 to 8 make it possible to switch thethree interfaces including USB, IEEE1394 and VGA, thus solving the sixthproblem. The VGA selector 601 or 800 and the IEEE1394 host selector 602or 801 can be integrally encased in a housing with the USB host selector600 or 100, respectively, without any problem.

A fifth embodiment of the invention will be described below.

FIG. 9 shows an embodiment for obviating the third and fourth problems.Due to the difference in internal configuration, the USB host selectoris designated by a different reference numeral 900 for apparentdistinction.

The feature of this USB host selector 900 lies in that as many USB businterfaces 107 as the USB connectors 101 associated with the USB hosts103 are included.

In the configuration example of FIG. 1 including only one USB businterface 107, a switching control command can be issued only from theUSB host 103 of which the connection is established. This is by reasonof the fact that the switching control section 110 is connected to aroute established through the USB bus interface 107.

In FIG. 9, on the other hand, USB hubs 105 are disposed immediatelyunder the USB connectors 104 associated with the USB hosts 103, and USBsignal lines 904 d, 904 f, 904 h branched from the respective USB hubs105 are applied to the USB bus interface 107. With this configuration, aswitching control command from an arbitrary USB host 103 can be appliedto a switching control section 901.

The configuration of the USB host selector 900 shown in FIG. 9 isapplicable to the USB host selectors 600 and 100 in FIGS. 5 and 8.

An example of the flow of operation of the switching control section 901for realizing the distinguished functions of the USB host selector 900will be explained with reference to FIG. 13.

After power is switched on, step E01 detects the receipt of a switchingcontrol command from at least a USB host 103. Step E02 is for preventinga connection route from being established to invalid USB hosts 103. Thefifth problem thus is solved.

In step E01, it may happen that a switching control command is receivedfrom a plurality of USB hosts 103 at the same time. Actually, therefore,various protective means such as exclusive control are required. Thisis, however, an implementation problem and will not be described herein.

As an application of this configuration, a cluster system made up ofeach USB host 103 is illustrated in the upper portion of FIG. 9. A faultmonitor software 907 constitutes part of the cluster control softwareand communicates with other USB hosts 103 through a network 911connected to the network interface 910 associated with each USB host.Each fault monitor software 907 notifies the other fault monitorsoftware 907 that it is “active”by sending a packet or the like means atregular intervals of time.

The control software 906 constitutes a feature of the present inventionand controls the USB host selector 900 in cooperation with the faultmonitor software 907. An example of the control operation flow of theUSB host selector 900 is shown in FIG. 12.

After power is switched on or otherwise, the USB host controller 908recognizes the USB host selector 900 (step C01), and then a CPU 909executes step C02 for initializing the USB host selector 900 inaccordance with the USB protocol.

Step C03 judges whether a signal is applied by key entry from thekeyboard constituting one of the USB devices 104, and if there is anykey entry, step C04 judges whether the key combination constitutes acommand for switching the USB host. Step C05 transmits a switchingcontrol command to the USB host selector 900.

Step C06 communicates with the fault monitor software 907 and detectswhether or not a fault has not occurred in any one of the USB hosts 103.If a fault is detected, step C07 transmits a switching control commandto the USB host selector 900 for switching to the USB host 103 sufferingfrom the fault. Alternatively, a switching control command can betransmitted to the USB host selector 900 for switching to the USB hosts103 not suffering from the fault.

The former switching process, in which the operation is switchedautomatically to the USB host 108 suffering from a fault, allows themanager to read the fault information quickly from the monitor and toinput required data from the keyboard to the USB host 103 suffering fromthe fault.

The latter switching process is based on the idea that switching to asystem in operation is better in view of the probable fact that the USBhost 103 suffering from a fault has nothing displayed on the monitor ora keyboard entry for a faulty USB host is invalid.

Which of the above processes to be selected can be specified by themanager giving a prior instruction to the control software 906. Such aselection can also depend on the degree of the particular fault.

As described above, a combination of the control software 900 and thefault monitor software 907 can realize a console switching operationinterlocked with a fault in any system including a cluster.

A sixth embodiment of the invention will be explained below.

Explanation will be made about a specific example of a configuration ofa switching section 109 included in the USB host selectors 100, 600, 900and the multi-host connection USB keyboard 400.

First, the problem involved will be explained again. Since the USB is adynamic insertion and removal interface, the USB devices 104 can beconnected or disconnected at an arbitrary timing with the USB host 103supplied with power.

The case in which a USB device 104 is connected will be described indetail. The USB devices 104 are supplied with power from the USB hosts103 through the USB cables and have the internal reset circuitactivated. This will hereinafter be called the hardware resetting. Inthe process, the USB device 104 connected sends a signal to thecorresponding USB host 103 indicating that the particular USB device 104is connected. Upon receipt of this signal, the USB host 103 sends areset command to the USB device 104 newly connected. The USB deviceinitializes the internal circuit conditions thereof. This operation ishereinafter referred to as the software resetting.

Generally, this initialization process consumes a considerable time.Suppose it takes several seconds from the time point when the USB hosts103 are switched by the USB host selector until an entry is possiblefrom the USB keyboard. The operating convenience on the part of theoperator would be considerably adversely affected.

In view of this, the present invention takes note of the fact that ifthe SUB device is subjected to the software resetting, the hardwareresetting is not necessarily required at the time of switching the USBhosts 103.

FIG. 10 shows an example of a block diagram showing a hardwareconfiguration of the switching section 109. First, explanation will bemade about a means for preventing the USB device 104 from beingsubjected to hardware resetting. The hardware resetting is effected inorder to initialize the internal circuits of the USB device 104 afterpower is switched on. The detection of the activation of the power line093 among the USB signal lines 904 provides a motive of hardwareresetting.

According to this invention, the power lines connected from all the USBhosts 103 are logically added by the circuit of diodes A07, and theresult is applied through the USB hub D 105 d to the USB device 104. Thediodes A07 are for preventing the shorting which otherwise might becaused if there exists a USB host 103 for which power is not switchedon.

As described above, power is kept supplied regardless of the switchingcontrol operation, and therefore the USB device is not subjected tohardware resetting.

Now, a means for causing the software resetting will be explained. TheUSB specification requires that whenever a new device is mounted, asignal indicating the mounting is applied to the USB hosts. In responseto this signal, the USB hosts 103 issue a software reset command.

According to this invention, a pseudo-mounting signal is applied to theUSB host 103 next to be connected before switching the route of a USBhost 103, and the returned software reset command is absorbed at theswitching section 109.

Specifically, a dynamic insertion and removal compensation circuit A00in FIG. 10 sends a signal procedure indicating the mounting of a devicethrough a D+ signal line A01 and a D− signal line A02 at the time ofswitching the USB hosts 103. Then, the connections A to C are switchedas actual connection routes. After that, the software reset command (acombination of the D+ signal line A01 and the D− signal line A02) issuedfrom a USB host 103 is applied to the USB device 104, thus executing theinitialization of the USB device.

FIG. 11 shows an example time chart. First, assume that a connectionroute A is established from the USB hub A 105 a to USB hub D 105 d (stepB10). The process for switching this route to the USB hub B 105 b, i.e.,to the connection route B will be explained.

FIG. 11 shows the lapse of time toward the right side thereof. First, instep B11, the USB host A 103 a of FIG. 9 issues a control command to theswitching control section 901 for switching to the connection route B.In step B12, the dynamic insertion and removal compensation circuit A00outputs a signal indicating the mounting of the USB device 104 to theUSB host B 103 b next to be connected. In response to this signal, theUSB host B 103 b sends a software reset command signal to the USB device104 in step B15.

The dynamic insertion and removal compensation circuit A00 detects thereset signal and switches the connection route from A to B (step B13).The reset signal from the USB host B 103 b is applied through the hub D105 d to the USB device 104. Step B16 initializes the USB device 104 andclears the intermediate result of the internal process as a result ofswitching the host.

After that, step B17 establishes the connection between the USB device104 completely reset and the new USB host B 103 b.

By going through the procedure described above, the USB hosts 103 can beswitched without any hardware resetting, thus solving the secondproblem.

In the USB specification, the signal procedure indicating the mountingof the USB device 104 and the reset command procedure from the USB hosts103 are described in the above-cited UNIVERSAL SERIAL BUS SPECIFICATIONversion 1.0 (Jan. 15, 1996), pp. 116-117, 119.

As described above, the interface switching apparatus and the interfaceswitching method according to the present invention have the first toseventh advantages described below.

A first advantage is that a USB device can be shared by a plurality ofUSB hosts.

A second advantage is that the hardware resetting of the UBS devicewhich otherwise might is required according to the USB specificationwhen switching the USB is eliminated, thereby reducing the time requireduntil the USB device becomes usable after switching.

A third advantage is that the USB device can be switched in an operationinterlocked with the failure of the cluster when a fault is detected ina host of the cluster system.

A fourth advantage is that a USB host can be switched without providingany new control interface between the USB host and the USB switch.

A fifth advantage is that the interface is prevented from being switchedto an input route not connected with a valid USB host when switching theUSB switching device, thereby improving the operating convenience of theuser.

A sixth advantage is that the interfaces such as VGA and IEEE1394 otherthan USB can be switched at a time when switching the USB interface.

A seventh advantage is that a single USB keyboard can be shared among aplurality of USB hosts having different architectures.

What is claimed is:
 1. A switching control method for a switch forswitching hosts from a first host to a second host with which a deviceis connected via a dynamic insertion and removal serial interface havinga power line for supplying electric power and a signal line fortransmitting a signal, said method performed in said switch comprisingthe steps of: receiving a control command for switching a connectionfrom said first host to said second host; securing said power to besupplied to said device; and switching said connection from said firsthost to said second host while said power is being supplied to saiddevice in response to said control command, wherein said step ofsecuring includes a substep of supplying said power to said device fromat least one of said first host and said second host.
 2. A switchingcontrol method according to claim 1, wherein said step of securingincludes a substep of changing the host that supplies said power to saiddevice from said first host to said second host.
 3. A switching controlmethod according to claim 1, further comprising the step of controllingsaid second host so as to issue a software reset command to said device.4. A switching control method according to claim 3, wherein said step ofcontrolling includes a substep of transmitting a pseudo-mounting signalfrom said switch to said second host.
 5. A switching control methodaccording to claim 4, wherein said step of controlling includes asubstep of transmitting from said switch a connection recognitionprotocol, which is the same signal transmitted by said device when newlysupplied with power, as said pseudo-mounting signal.
 6. A switchingcontrol method according to claim 1, wherein a universal serial bus isused as said serial interface.
 7. A switching control method for aswitch for switching hosts from a first host to a second host with whicha device is connected via a dynamic insertion and removal serialinterface having a power line for supplying electric power and a signalline for transmitting a signal, said method performed in said switchcomprising the steps of: supplying said power to said devicecontinuously; receiving a control command for switching a connectionfrom said first host to said second host; and switching said connectionfrom said first host to said second host in response to said controlcommand, wherein the step of supplying includes a substep of supplyingsaid power to said device from at least one of said first host and saidsecond host.
 8. A switching control method according to claim 7, whereinthe step of supplying includes a substep of securing said power bychanging the host that supplies said power to said device from saidfirst host to said second host.
 9. A switching control method accordingto claim 7, further comprising the step of controlling said second hostso as to issue a software reset command to said device.
 10. A switchingcontrol method according to claim 9, wherein the step of controllingincludes a substep of transmitting a pseudo-mounting signal from saidswitch to said second host.
 11. A switching control method according toclaim 10, wherein the step of controlling includes a substep oftransmitting from said switch a connection recognition protocol, whichis the same signal transmitted by said device when newly supplied withpower, as said pseudo-mounting signal.
 12. A switching control methodaccording to claim 7, wherein a universal serial bus is used as saidserial interface.
 13. A switching control method for a switch forswitching hosts from a first host to a second host with which a deviceconnects via a dynamic insertion and removal serial interface having apower line for supplying power and a signal line for transmitting asignal, said method performed in said switch comprising the steps of:receiving a control command for switching a connection from said firsthost to said second host; supplying said power to said device from atleast one of said second host and said first host; and switching saidconnection from said first host to said second host while said power isbeing supplied to said device in response to said control command.
 14. Aswitching control method according to claim 13, wherein the step ofsupplying includes a substep of securing said power by changing the hostthat supplies said power to said device from said first host to saidsecond host.
 15. A switching control method according to claim 13,further comprising the step of controlling said second host so as toissue a software reset command to said device.
 16. A switching controlmethod according to claim 15, wherein said step of controlling includesa substep of transmitting a pseudo-mounting signal from said switch tosaid second host.
 17. A switching control method according to claim 16,wherein said step of controlling includes a substep of transmitting fromsaid switch a connection recognition protocol, which is the same signaltransmitted by said device when newly supplied with power, as saidpseudo-mounting signal.
 18. A switching control method according toclaim 12, wherein a universal serial bus is used as said serialinterface.